U.S. health regulators on Monday approved Gilead Sciences Inc's four-drug combination pill to treat HIV, the virus that causes AIDS. The four drugs in one pill, which was formerly called the Quad and had been considered one of Gilead's more important future growth drivers, will be sold under the brand name Stribild, the U.S. Food and Drug Administration said. Gilead shares were virtually unchanged from their $57.19 Nasdaq close as Wall Street has been focused on development of the experimental hepatitis C treatment that Gilead acquired with its $11 billion purchase of Pharmasset.